Methods and devices for ac current sources, precision current transducers and detectors

ABSTRACT

Precision AC voltage, current, phase, power and energy measurements and calibrations with current ranges from 1 uA to 20 kA and voltage ranges from 1V to 1000 kV are now performed with accuracies of better than one part per million. Continued demand for improved accuracy has led the inventors to address improvements to dual stage and multi-stage current transducers that may form the basis of the measuring process within many of the measurement instruments providing the precision AC measurements and calibrations. Additionally, the improvements to dual stage and multi-stage current transducers provide for novel feedback controlled precision AC current sources without requiring measurement of the AC current source output directly.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims the benefit of priority as a continuationof U.S. patent application Ser. No. 14/663,580 filed Mar. 20, 2015entitled “Methods and Devices for AC Current Sources, Precision CurrentTransducers and Detectors” which itself claims priority from U.S.Provisional Patent Application U.S. 61/968,557 filed Mar. 21, 2014entitled “Methods and Devices for AC Current Sources, Precision CurrentTransducers and Detectors,” the entire contents of both are incorporatedherein by reference.

FIELD OF THE INVENTION

This invention relates generally to precision AC current sources,precision current transducers, and measurements, which include precisionAC current, voltage, phase, impedance, frequency, power and energymeasurements, over current ranges from 1 mA or less to 20 kA or greaterand voltage ranges of 1V or less to 1000 kV or greater and overfrequency ranges from a few hertz to hundreds of kilohertz. Inparticular it relates to precision AC current sources, precision currenttransducers, and measurements using enhanced dual stage currenttransducers.

BACKGROUND OF THE INVENTION

Alternating Current (AC) electrical measurements are used in a widevariety of applications and may be performed for a variety of electricalquantities including, for example, voltage, current, capacitance,impedance, frequency, phase, power, energy, and resistance. These testsand measurements include those relating to designing, evaluating,maintaining and servicing electrical circuits and equipment range fromhigh voltage electrical transmission lines operating at hundreds ofkilovolts (kV) and kiloamps (kA) to industrial/medical/residentialelectrical and lighting, typically 400V/240V/100V and 30/15 A, to a widevariety of industrial/scientific/medical/consumer electrical andelectronic devices operating at voltages of hundreds of mV and currentsof a few mA.

Within a variety of AC current applications and AC current testequipment systems AC comparator bridges and AC current transformers areemployed to provide the required dynamic range, accuracy, andflexibility. AC current bridge configurations remove many of the issuesassociated with achieving making measurements at accuracies of a part,or few parts per million, such as insensitivity to lead resistances,excellent ratio linearity, excellent ratio stability, and a high levelof resolution. AC current transformers, importantly, isolate themeasuring instruments from what may be very high voltage in themonitored circuit and when the current in a circuit is too high to bedirectly applied to measuring instruments, a current transformerproduces a reduced current accurately proportional to the current in thecircuit, which can be conveniently connected to measuring and recordinginstruments. They also allow accurate high current generation fromprecision lower current sources and isolation of the precision sourcefrom external variations.

Accordingly many sources and measurement systems for alternating currentpower systems have a current transformer at their output and inputstages respectively. Over the past approximately 180 years whilst a widevariety of types of electrical transformer are made for differentpurposes these, despite their design differences, employ the same basicprinciple as discovered in 1831 by Michael Faraday, and share severalkey functional parts. Over this period many techniques have beendeveloped to improve the accuracy of the current transformer. Amongthem, the dual stage current transformer, described in the work ofBrooks and Holtz in “The Two-Stage Current Transformer” (AIEE Trans.,Vol. 41, pp 382-393, 1922) still forms the basis for a significantproportion of commercial systems. These transformers are generally whatis referred to as “step down transformers” for converting highvoltage—low current inputs to lower voltage—higher current outputs.

However, in a range of other applications within electrical systems andmeasurement systems what is required are precision AC current sourcesand AC amplifiers. The inventors have found that improvement of theaccuracy when designing a precision AC current source is a differentproblem to measurement systems in that we either wish to removemeasuring equipment connected to the output circuit to provide thefeedback or wish that the generation and measurement of even very largecurrent AC current sources is performed without requiring the use of ashunt.

Accordingly, the inventors have established design and circuitmethodologies which are applicable to precision AC current sources,amplifiers, and also AC current measurements. Such measurements includeprecision AC current, voltage, phase, impedance, frequency, power andenergy measurements, over current ranges from 1 mA or less to 20 kA orgreater and voltage ranges of 1V or less to 1000 kV or greater and overfrequency ranges from a few hertz to hundreds of kilohertz. Similarly,precision AC current sources and amplifiers for test, measurement, andsupply applications are desirable over current ranges from 1 mA or lessto 20 kA or greater and voltage ranges of 1V or less to 1000 kV orgreater and over frequency ranges from a few hertz to hundreds ofkilohertz.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide improvements overthe prior art in respect of AC precision current sources, amplifiers,and measurements, which include precision AC current, voltage, phase,impedance, frequency, power and energy measurements, over current rangesfrom 1 mA or less to 20 kA or greater and voltage ranges of 1V or lessto 1000 kV or greater and over frequency ranges from a few hertz tohundreds of kilohertz. In particular it relates to precision AC currentsources, precision current transducers, and measurements using enhanceddual stage current transducers.

In accordance with an embodiment of the invention there is provided adevice comprising:

-   a dual stage current transducer comprising at least a primary    winding, a first secondary winding and a second secondary winding;-   a first four terminal shunt coupled across the first secondary    winding; and-   a second four terminal shunt coupled across the second secondary    winding; wherein-   a first voltage generated across the second four terminal shunt is    subtracted from a second voltage generated across the first four    terminal shunt.

In accordance with an embodiment of the invention there is provided adevice comprising:

-   a dual stage current transducer comprising at least a primary    winding, a first secondary winding and a second secondary winding;-   a first four terminal shunt coupled across the first secondary    winding;-   a second four terminal shunt coupled across the second secondary    winding;-   an alternating current source disposed between the first secondary    winding and first four terminal shunt; and-   a third four terminal shunt coupled in series with a load across the    primary winding.

In accordance with an embodiment of the invention there is provided amethod comprising providing a multi-stage current transducer with afirst means to obtain a first voltage proportional to a primary currentof said multi-stage current transducer and a second means to obtain asecond voltage proportional to a secondary current in a second stage ofthe multi-stage current transducer, said secondary current beingproportional to the magnetizing current of the magnetic core of a firststage of the multi-stage current transducer.

In accordance with an embodiment of the invention there is provided amethod comprising:

-   providing a current transducer having two stages where current of a    first secondary of the current transducer passes through a first    four terminal shunt and a current of a second secondary of the    current transducer passes through a second four terminal shunt;-   summing the voltages from the first and second four terminal shunts    to represent the instantaneous value of the primary current within    the current transducer; and-   at least one of:    -   digitizing the resulting summed voltage; and    -   providing the current transducer comprises providing a first        magnetic core of the current transducer in the form of a hollow        toroid and a second magnetic core of the current transducer in        the form of a toroid core embedded within the first magnetic        core.

In accordance with an embodiment of the invention there is provided amethod comprising providing a bridge for establishing the value of theresistance and the inductance of a load, the bridge comprising a currenttransducer having two stages and first to third four terminal shunts,wherein a first current within a first secondary of the currenttransducer passes through a first four terminal shunt, a second currentwithin a second secondary of the current transducer passes through asecond four terminal shunt and a third current passing through the loaddisposed across a primary of the current transducer also passes throughthe third four terminal shunt.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the attached Figures, wherein:

FIG. 1 depicts an AC current transformer according to the prior artusing a dual stage current transformer design of Brooks and Holtz;

FIG. 2A depicts a circuit using a dual stage AC current transducer withelectrical shield and four terminal resistor for improved accuracy;

FIG. 2B depicts a circuit using dual stage AC current transducers with atransimpedance amplifier to sum the currents from the first and secondstages;

FIG. 3A depicts an AC dual stage current transducer using dualindependent resistors for improved accuracy according to an embodimentof the invention;

FIG. 3B depicts a precision AC current source exploiting a dual stagecurrent transducer, dual independent load resistors, and a programmablecontrolled current source according to an embodiment of the invention;

FIG. 4A depicts a precision AC current source exploiting a dual stagecurrent transducer, dual independent load resistors, and with anadjustable uncontrolled current source according to an embodiment of theinvention transducer;

FIG. 4B depicts a precision AC current source exploiting a dual stagecurrent transducer with adjustable uncontrolled current source and atransimpedance amplifier to sum the currents from the first and secondstages according to an embodiment of the invention;

FIGS. 5A through 5C depict high current AC shunt calibrators accordingto embodiments of the invention exploiting a dual stage currenttransducers

FIG. 6 depicts dual stage current transducer designs exploitingcore-in-core, dual core, and triple core designs to provide AC devicesaccording to embodiments of the invention as described in respect ofFIGS. 2A through 5B;

FIG. 7 depicts a dual stage current transducer design exploiting acore-in-core design to provide AC devices according to embodiments ofthe invention as described in respect of FIGS. 2A through 5B; and

FIG. 8 depicts a dual stage current transducer design exploiting a threecore design to provide AC devices according to embodiments of theinvention as described in respect of FIGS. 2A through 5B.

DETAILED DESCRIPTION

The present invention is directed to improvements over the prior art inrespect of AC precision current sources, precision current transducers,and measurements, which include precision AC current, voltage, phase,impedance, frequency, power and energy measurements, over current rangesfrom 1 mA or less to 20 kA or greater and voltage ranges of 1V or lessto 1000 kV or greater and over frequency ranges from a few hertz tohundreds of kilohertz. In particular it relates to precision AC currentsources, precision current transducers, and measurements using enhanceddual stage current transducers.

The ensuing description provides exemplary embodiment(s) only, and isnot intended to limit the scope, applicability or configuration of thedisclosure. Rather, the ensuing description of the exemplaryembodiment(s) will provide those skilled in the art with an enablingdescription for implementing an exemplary embodiment. It beingunderstood that various changes may be made in the function andarrangement of elements without departing from the spirit and scope asset forth in the appended claims.

Within the drawings presented in respect of this specification elementshaving the same number are the same element and may or may not bereferenced explicitly in every drawing due to the recurring elementsbeing commonly numbered.

Dual stage transformers as known by one of skill in the art comprisethree windings together with one or more magnetic cores. These threewindings are commonly referred to as the primary winding, to which thesignal to be transformed by the dual stage transformer is coupled, thefirst secondary winding, from which the transformed signal is coupled,and the second secondary winding (also known as the correction winding),from which a signal (commonly referred to as the correction signal) iscoupled. The primary winding and first secondary winding may in someembodiments of the dual stage transformer be conceptually identical andcoupled with the same magnetic flux and can be, for example, swapped toreverse the dual stage transformer operation. In contrast the secondsecondary winding cannot be swapped with either of the main windings,namely the primary winding and first secondary winding. Within thisdocument, except for the claims and the summary of the invention wherethe terms first secondary winding and second secondary winding aremaintained, the first secondary winding will be referred to as the“secondary winding” (with the current flowing within it referred to asthe secondary current) and the second secondary winding will be referredto as the correction winding (with the current flowing within itreferred to as the correction current).

Referring to FIG. 1 there is depicted a dual stage transformer 100according to the prior art of Brooks and Holtz, see “The Two-StageCurrent Transformer” (AIEE Trans., Vol. 41, pp 382-393, 1922). Withinthis the current transformation is effected in two stages, the firstgenerated by secondary winding N₁ 110B in response to the signal coupledto the primary winding N₀ 110A, which is approximately correct inmagnitude and phase. The second stage is the generation of an auxiliarycorrective current via correction winding N₂ 110C which, when combinedwith the secondary current, gives a resultant current which very closelyapproximates to the secondary current which would be furnished by anideal current transformer having no errors. As depicted the secondaryand correction windings 110B and 110C respectively are coupled acrossload resistor 120 generating a potential across first and second outputs100A and 100B respectively which are coupled to circuit 130, which forexample contains one or more analog-to-digital converters (ADCs) as partof measuring the converted signal. According to the ratio of the turnsin the primary winding 110A to the secondary and correction windings110B and 110C the resulting output may be scaled up from the inputsignal, scaled down, or even be simply equal such that the measurementcircuit, e.g. circuit 130, is buffered from the input signal carryingcircuit.

Referring to FIG. 2A there is depicted a dual stage transducer 200Awherein a current transducer CT_(R) 2000 again comprises primary winding2000A and secondary and correction windings 2000B and 2000C but now ashield 130 is disposed between the primary winding 2000A and themagnetic core of the current transducer 2000 and coupled via shield portSh 200C to circuit 230. Also within dual stage transducer 200 the loadresistor 120 is replaced by a non-inductive four terminal shunt R 210thereby increasing the accuracy of the reproduced voltage across thesignal output ports 200A and 200B respectively. The non-inductive fourterminal shunt R 210 may, for example, be a “Kelvin” configurationresistor with four terminals (via leads) allowing a current to beapplied though a pair of opposite leads and the voltage to be sensedacross the other pair of opposite leads. The “Kelvin” configurationeffectively eliminates the resistance and temperature dependence of theleads.

Referring to FIG. 2B there is depicted a dual stage transducer 200Bwherein a current transducer CT_(R) 2000 again comprises primary winding2000A and secondary and correction windings 2000B and 2000C respectivelywith the shield 130 disposed between the primary winding 2000A and themagnetic core of the current transducer 2000 and coupled via shield portSh 200C to circuit 230. However, in dual stage transducer 200B thenon-inductive four terminal shunt R 210 employed within dual stagetransducer 200A in FIG. 2A is replaced by a transimpedance amplifier(TIA) 250 with feedback resistor 220. As depicted one side of each thesecondary and correction windings 200A and 200B respectively are coupledto the positive input port of the TIA 250 whilst the negative input portof the TIA 250 is coupled to the other side of each the secondary andcorrection windings 200A and 200B respectively. The output of the TIA250 being coupled to first output port 200A whilst the sides of thesecondary and correction windings 200A and 200B respectively are coupledto the second output port 200B.

However, in many test and measurement applications even the enhancedcurrent reproduction and error reduction of dual stage transducers 200Aand 200B is insufficient. The continued drive for improved accuracy incalibration, standards, and measurements on circuits and componentsoperating at hundreds of kilovolts, thousands of Amps, with resistancesinto Gigaohms at accuracies of parts per million is being replaced byaccuracies of parts per billion. Accordingly, referring to FIG. 3A thereis depicted a dual stage transducer 300 according to an embodiment ofthe invention employing a CT_(R) 2000 in conjunction with first andsecond four (4) terminal resistors (4TeR) 310 and 320 respectively. Asdepicted the secondary current from the secondary winding 2000B isconnected to the first 4TeR 310 and the second secondary current fromthe correction winding 2000C is connected to the second 4TeR 320, e.g.the magnetizing current of the first stage transducer is coupled to thisresistor. If, now the first and second 4TeR 310 and 320 are seriallyconnected on their voltage measurement terminals then the sum of thesetwo voltages are an accurate replica of the current being measured. Thisarises in part due to the fact, that the required induced voltage in thesecond core of the CT_(R) 2000 is much smaller and consequently theremaining error of the magnetizing current of the second stage isnegligible.

Within some embodiments of the invention the second stage (correction)current and voltage within the dual stage transducer are small andaccordingly, depending upon the precision of the source, measurementcircuit, etc. that they form part of, the precision 4TeR 320 may bereplaced with a suitably tolerance two terminal resistor.

Optionally, to obtain an even more accurate voltage proportional to themagnetizing current of the second stage an amplifier, e.g. an electronicamplifier, may be employed such that the voltage across the correctionwinding 2000C is reduced even further. Accordingly, the error due to themagnetizing current of the second stage, which is related to the voltagedrop on the impedance of that correction winding, denoted Z₂, isnegligible because this current is small but the error due to thevoltage on the prior art four-terminal shunt resistor R 210 issignificant.

In addition to improved accuracy in calibration, standards, andmeasurements on circuits and components arising from the measurementcircuits themselves a corresponding drive in improved accuracy exists inthe design and implementation of precision sources of alternatingcurrent within test and measurement instrumentation. Whilst this mayappear a different problem to that of the measurement circuit theinventors have realized that actually the technique to solve it issimilar to that depicted in FIG. 3A in respect of enhanced accuracycurrent measurements. Accordingly, referring to FIG. 3B there isdepicted a precision AC current source (PACCS) 350 according to anembodiment of the invention exploiting a current transducer CT_(R) 2000such as described supra in respect of FIG. 3A. Accordingly, a controlledcurrent source 410 has been inserted into the circuit loop comprisingsecondary winding 2000B and first 4TeR R1 310. The controlled currentsource 410 is coupled to the control circuit 430 via control port 400C.Beneficially the PACCS 350 allows a voltage proportional to the outputcurrent of an AC transconductance amplifier to be precisely obtainedwithout the measuring equipment being connected to the output circuit.As the controlled current source 410 is coupled to the secondary winding2000B and the load Z 420 is coupled across the primary winding 2000Athen strictly the secondary winding 2000B is the “primary winding” andprimary winding 2000A the “secondary winding” of the PACCS 350 whilstthe correction winding is essentially unchanged.

Accordingly, feedback information for the regulation of the controlledcurrent source 410 within the PACCS 350 is derived from the output ofthe PACCS 350, this being the voltage on the first 4TeR R1 310 fromwhich is subtracted the voltage on the second 4TeR R2 320 generated bythe current flowing within a second current loop comprising second 4TeRR2 320 and correction winding 2000C of the CT_(R) 2000. Accordingly,this output voltage V across terminals 400A and 400B is proportional tothe output current and hence can be used as feedback information. Itwould be evident that measuring this output voltage V using ananalog-to-digital converter (ADC) would allow the value of the outputsecondary current to be obtained in digital form for use within adigital feedback loop to the programmable current source 410.Alternatively, an analog feedback loop may be employed but it should beemphasized that in either instance the current is measured withoutconnecting any measuring device in the output circuit, a verysignificant feature against prior art precision current sources withfeedback. Further, the problem of generating and measuring even verylarge currents are addressed without the requirement for using shunts.

Within some embodiments of the invention, such as depicted by first andsecond PACCS 400A and 400B respectively in FIGS. 4A and 4B, thecontrolled current source 410 which is part of a feedback control loopfor the PACCS 350 may be replaced by adjustable and programmable ACcurrent sources 440 and 460 respectively. As depicted in FIG. 4A firstPACCS 400A which exploits first 4TeR R1 310 and second 4TeR R2 320 inconjunction with adjustable AC current source 440 is not coupled to thecontrol circuit 430. However, the signal level of the adjustable ACcurrent source 440 may be set, thereby setting the output currentsupplied to load Z 420, such that subsequently the value of the currentis measured, and this is then used for calibration by the controlcircuit 430. In contrast in FIG. 4B second PACCS 400B exploitsprogrammable AC current source 460 which is programmed via a digitalcontrol word through data port 400C allowing the control circuit 430 toestablish multiple settings for the PACCS 400B. Accordingly, the load Z420 is driven at multiple output currents/voltages and feedback to thecontrol circuit 450 is achieved through TIA 250.

Accordingly, referring to FIGS. 3B, 4A and 4B there are depictedprecision AC current sources (PACCS) 350, 400A and 400B respectivelyaccording to an embodiment of the invention exploiting a currenttransducer CT_(R) 2000 such as described supra. Beneficially each ofPACCS 350, 400A, and 400B allow a voltage proportional to the outputcurrent of the PACCS to be precisely obtained without requiring thatprecision measuring equipment is connected to the output circuittogether with the load Z 420s.

The embodiments of the invention described above in respect of FIGS. 2Athrough 4B assume that the voltage induced in the uniformly wound coilon the toroidal magnetic core of the CT_(R) 2000 is proportional to thetotal ampere-turns passing through the opening of that magnetic core andconsequently that only the magnetizing current is causing the error.However, with the development of the current comparator, see for exampleMiljanic et al. in “The Development of the Current Comparator: A HighAccuracy AC Ratio Measuring Device” (IEEE Part 1: Comm. & Elect., Vol81(5), pp 359-368), it was shown that the voltage induced in the windingwound on the toroidal magnetic core measures the total ampere-turnpassing through its opening only if it is shielded from stray magneticand electric fields. Accordingly, for embodiments of the invention asdescribed in respect of FIG. 6 below a shield, for example a hollowtoroid of the magnetic material which surrounds the measuring coresituated in its interior and/or a copper tape/box for electricalshielding.

It would be evident to one skilled in the art that the PACCS 400 may beconsidered as a combination of a dual stage current transducer and ashielded current comparator wherein the magnetic shield of the currentcomparator is used as the magnetic core of the first stage of the dualstage current transducer, and the detection winding of the currentcomparator is actually the second stage of the dual stage currenttransducer.

Referring to FIG. 5A there is depicted an AC shunt calibrator 550A,particularly for their calibration at high current. Accordingly, asdepicted a PACCS 5000, which is depicted as PACCS 350 in FIG. 3B withcontrolled current source 410 disposed within the circuit comprising thesecondary winding of a CT_(R) 2000 in conjunction with first 4TeR R1310. The calibration AC shunt R3 510 is a 4 terminal resistor whereinthe other terminals are coupled to the control circuit 530 as Out 1 andOut 2 at ports 500A and 500B respectively. The H and L outputs fromPACCS 350 are now depicted as Out 3 and Out 4 at ports 500C and 500Drespectively which are also coupled to the control circuit 530. Alsoconnected to PACCS 350 from the control circuit 530 are Shy 200C for theshield within CT_(R) 2000 and control 400C whilst a microprocessor 540is coupled to the control circuit 530. Accordingly, the AC shuntcalibrator 550A can be calibrated over a range of test conditions,established through the setting of the controlled current source 410under the action of the control circuit 530, by determining the currentvia Out 3 and Out 4 on ports 500C and 500D together with the voltageacross the calibration AC shunt 510 via Out 1 and Out 2 on ports 500Aand 500B. These may be measured using two independent voltmeters (orADCs). This configuration provides flexibility in where and if a commonground connection is made.

Now referring to FIG. 5B there is depicted an AC shunt calibrator 550Baccording to an embodiment of the invention wherein PACCS 5500, which isdepicted as comprising PACCS 400B in FIG. 4B except that theprogrammable AC current source 460 has been replaced with adjustable ACcurrent source 440 such that there is no control/data signal to thecurrent source within the PACCS 5500 from the control circuit 530.Referring to FIG. 5C there is depicted a variant of AC shunt calibrator550A in AC shunt calibrator 550C wherein output Out 3 500C, coupled tofirst 4TeR R1 310 is still coupled to control circuit 530 but isgrounded. However, now Out 4 500D, which is coupled to second 4TeR R2320, is coupled with Out 2 500B and therein the control circuit 530rather than directly to the control circuit. Accordingly, variations inthe output of the PACCS 5000 which are applied to the Load Z 420 undertest are automatically applied to the measured current flowing in theload circuit via third 4TeR R3 510.

Within the embodiments of the invention depicted supra in respect ofprecision AC current sources FIGS. 3A, 4A and 4B and exploited withinthe AC shunt calibrators 550A to 550C in FIGS. 5A to 5C respectivelycurrent sources are employed in conjunction with the secondary winding.However, in other embodiments of the invention these current sources maybe an AC power source in order to drive power shunts during calibration.Such AC power sources may range from 50 W to 1000 W, for example.

Referring to FIG. 6 there are depicted first to third dual stage currenttransducers (2SCT_(R)) 600A to 600C respectively designs exploitingcore-in-core, dual core, and triple core designs respectively to provideAC devices according to embodiments of the invention as described inrespect of FIGS. 2 through 5. Referring to first 2SCT_(R) 600A amagnetic shield, the first stage magnetic core 610, has wound around itprimary winding 620 and secondary winding 630. Disposed within themagnetic shield are electric shield 650 and correction winding 640 whichsurround measuring toroid, second stage magnetic core 660.

Second 2SCT_(R) 600B depicts the same elements except that now the firststage magnetic core 610 and second stage magnetic core 660 are a pair ofparallel toroids wherein the primary winding 620 and secondary winding630 surround both as does the electrical shield 650. The correctionwinding 640 then surrounds only the second stage magnetic core 660.Similarly in third 2SCT_(R) 600C depicts the same elements except thatnow the first stage magnetic core comprises first and second coreelements 610A and 610B respectively and these, in conjunction with thesecond stage magnetic core 660 are a triplet of parallel toroids.Accordingly, in third 2SCT_(R) 600C the primary winding 620 andsecondary winding 630 surround the first and second core elements 610Aand 610B and second stage magnetic core 660. The electrical shield 650surrounds only the second stage magnetic core 660 as does the correctionwinding 640. Other embodiments of a 2SCT_(R) may be envisioned withoutdeparting from the scope of the invention.

FIG. 7 depicts a dual stage current transducer (2SCT_(R)) designexploiting a core-in-core design to provide AC devices according toembodiments of the invention as described in respect of FIGS. 2 through5 and first 2SCT_(R) 600A. Accordingly first image 700C depicts the2SCT_(R) sequentially stripped from the outermost layer towards thecentre whilst second image 700D depicts a three dimensional quarter-cutsectional view with first to fifth tape layers 730A through 730Erespectively and shielding 760 removed for clarity. Accordingly as shownthe 2SCT_(R) comprises a first core comprising first to fourth coreelements 710A to 710D respectively surround a second core 720. Secondcore 720 then has first tape layer 730A separating the first winding 740from it which is then overwound with second tape layer 730B. The firstcore (first to fourth core elements 710A to 710D) and second core 720respectively with their respective surrounding layers are then overwoundwith third tape layer 730C. Atop third tape layer 730C second winding750 is wound around first core (first to fourth core elements 710A to710D) and second core 720. Second winding 750 is then overwound byfourth tape layer 730D, shielding 760, fifth tape layer 730E and thirdwinding 770.

As depicted first winding 740 corresponds to correction winding N₂ ofFIG. 10, second winding 750 corresponds to secondary winding N₁ of FIG.10, and third winding 770 corresponds to the primary winding N₀ of FIG.10. Optionally a second shielding may be disposed between the first andsecond windings 740 and 750 respectively such as between second andthird tape layers 730B and 730C respectively. Second image 700D depictsa three dimensional quarter-cut sectional view with first to fifth tapelayers 730A through 730E respectively and shielding 760 removed therebyshowing how the first to third windings 740, 750 and 770 respectivelyare wound around the closed magnetic elements forming the first, second,and third cores 710A, 720, and 710B respectively. It would be evident toone skilled in the art that the number of windings for each of the firstto third windings 740, 750, and 770 respectively and geometries of thefirst core (first to fourth core elements 710A to 710D) and second core720 respectively may be adjusted according to the electrical voltage,current and power of the signal being measured and/or generated.

FIG. 8 depicts a dual stage current transducer (2SCT_(R)) designexploiting a three-core design to provide AC devices according toembodiments of the invention as described in respect of FIGS. 2 through5 and third 2SCT_(R) 600C. Accordingly first image 800C depicts the CTsequentially stripped from the outermost layer towards the centre whilstsecond image 800D depicts a three dimensional quarter-cut sectional viewwith first to fifth tape layers 830A through 830E respectively andshielding 860. Accordingly as shown the CT comprises first, second, andthird cores 810A, 820, and 810B respectively. Second core 820 then hasfirst tape layer 830A separating the first winding 840 from it which isthen overwound with second tape layer 830B. The first, second, and thirdcores 810A, 820, and 810B respectively with their respective surroundinglayers are then overwound with third tape layer 830C. Atop third tapelayer 830C second winding 850 is wound around first, second, and thirdcores 810A, 820, and 810B respectively. Second winding 850 is thenoverwound by fourth tape layer 830D, shielding 860, fifth tape layer830E and third winding 870. As depicted first winding 840 corresponds tocorrection winding N₂ of FIG. 10, second winding 850 corresponds tosecondary winding N₁ of FIG. 10, and third winding 870 corresponds tothe primary winding N₀ of FIG. 10. Optionally a second shielding may bedisposed between the first and second windings 840 and 850 respectivelysuch as between second and third tape layers 830B and 830C respectively.

Second image 800D depicts a three dimensional quarter-cut sectional viewwith first to fifth tape layers 830A through 830E respectively andshielding 860 removed thereby showing how the first to third windings840, 850 and 870 respectively are wound around the closed magneticelements forming the first, second, and third cores 810A, 820, and 810Brespectively. It would be evident to one skilled in the art that thenumber of windings for each of the first to third windings 840, 850, and870 respectively and geometries of the first, second, and third cores810A, 820, and 810B respectively may be adjusted according to theelectrical voltage, current and power of the signal being measuredand/or generated.

Specific details are given in the above description to provide athorough understanding of the embodiments. However, it is understoodthat the embodiments may be practiced without these specific details.For example, circuits may be shown in block diagrams in order not toobscure the embodiments in unnecessary detail. In other instances,well-known circuits, processes, algorithms, structures, and techniquesmay be shown without unnecessary detail in order to avoid obscuring theembodiments.

The foregoing disclosure of the exemplary embodiments of the presentinvention has been presented for purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise forms disclosed. Many variations andmodifications of the embodiments described herein will be apparent toone of ordinary skill in the art in light of the above disclosure. Thescope of the invention is to be defined only by the claims appendedhereto, and by their equivalents.

Further, in describing representative embodiments of the presentinvention, the specification may have presented the method and/orprocess of the present invention as a particular sequence of steps.However, to the extent that the method or process does not rely on theparticular order of steps set forth herein, the method or process shouldnot be limited to the particular sequence of steps described. As one ofordinary skill in the art would appreciate, other sequences of steps maybe possible. Therefore, the particular order of the steps set forth inthe specification should not be construed as limitations on the claims.In addition, the claims directed to the method and/or process of thepresent invention should not be limited to the performance of theirsteps in the order written, and one skilled in the art can readilyappreciate that the sequences may be varied and still remain within thespirit and scope of the present invention.

What is claimed is:
 1. A current source comprising: a dual stage currenttransducer comprising at least a primary winding, a first secondarywinding and a second secondary winding; an alternating current sourcedisposed between the first secondary winding and a processing circuitfor generating a current to generate a first electrical output signalacross the primary winding to be coupled to a load and comprising acontrol port for receiving a control signal for controlling thealternating current source; a processing circuit coupled to the firstsecondary winding and the second secondary winding for providing asecond electrical output signal relating to the current generated by thealternating current source to an external circuit; and a control circuitcoupled to the processing circuit for receiving the second electricaloutput signal and generating the control signal in dependence upon atleast the second electrical output signal.
 2. The current sourceaccording to claim 1, wherein the processing circuit comprises a firstfour terminal shunt and a second four terminal shunt; wherein a firstcurrent lead of the first four terminal shunt is coupled to a firstpredetermined end of the first secondary winding and a second currentlead of the first four terminal shunt is coupled to the other end of thefirst secondary winding via the alternating current source; a firstcurrent lead of the second four terminal shunt is coupled to a first endof the second secondary winding corresponding to the first end of thefirst secondary winding and a second current lead of the second fourterminal shunt is coupled to the other end of the second secondarywinding; a first voltage lead of the first four terminal shunt at thesame end as the first current lead of the first four terminal shunt iscoupled to the external circuit; a first voltage lead of the second fourterminal shunt at the same end as the second current lead of the secondfour terminal shunt is coupled to the external circuit; and the secondvoltage leads of the first and second four terminal shunts are coupledtogether.
 3. The current source according to claim 1, wherein theprocessing circuit comprises a current comparator having a first input,a second input, and an output; and a resistor disposed between the firstinput of the current comparator and the output of the current comparatorwhich is also coupled to the external circuit; wherein the first inputof the current comparator is coupled to a first predetermined end of thefirst secondary winding and a first predetermined end of the secondsecondary winding corresponding to the first end of the first secondarywinding; the second input of the current comparator is coupled to theother end of the second secondary winding and to the other end of thefirst secondary winding via the alternating current source.
 4. Thecurrent source according to claim 1, wherein the alternating currentsource is selected from the group comprising a stabilized alternatingcurrent source, an adjustable alternating current source, a programmablealternating current source and a controllable alternating currentsource.
 5. The current source according to claim 1, further comprising athird four terminal shunt coupled in series with a load across theprimary winding; and the processing circuit comprises a first fourterminal shunt and a second four terminal shunt; wherein a first currentlead of the first four terminal shunt is coupled to a firstpredetermined end of the first secondary winding and a second currentlead of the first four terminal shunt is coupled to the other end of thefirst secondary winding via the alternating current source; a firstcurrent lead of the second four terminal shunt is coupled to a first endof the second secondary winding corresponding to the first end of thefirst secondary winding and a second current lead of the second fourterminal shunt is coupled to the other end of the second secondarywinding; a first current lead of the third four terminal shunt iscoupled to the primary winding at the end corresponding to the first endof the first secondary winding and a second current lead of the thirdfour terminal shunt is coupled to the other end of the primary windingvia the load; a first voltage lead of the first four terminal shunt atthe same end as the first current lead of the first four terminal shuntis coupled to ground and the external circuit; a first voltage lead ofthe second four terminal shunt is coupled to a second voltage lead ofthe second terminal shunt at the same end as the second current lead ofthe second terminal shunt and the external circuit; a first voltage leadof the second four terminal shunt at the same end as the second currentlead of the second four terminal shunt is coupled to the externalcircuit; the second voltage leads of the first and second four terminalshunts are coupled together; a first voltage lead of the third fourterminal shunt at the same end as the first current lead of the thirdfour terminal shunt is coupled to the external circuit; and a secondvoltage lead of the second four terminal shunt is coupled to a secondvoltage lead of the third four terminal shunt and to the externalcircuit.
 6. The current source according to claim 1, further comprisingthe processing circuit comprises a current comparator having a firstinput, a second input, and an output; a resistor disposed between thefirst input of the current comparator and the output of the currentcomparator which is also coupled to the external circuit; and a fourterminal shunt coupled in series with a load across the primary winding;wherein the first input of the current comparator is coupled to a firstpredetermined end of the first secondary winding and a firstpredetermined end of the second secondary winding corresponding to thefirst end of the first secondary winding; the second input of thecurrent comparator is coupled to the other end of the second secondarywinding and to the other end of the first secondary winding via thealternating current source; a first current lead of the third fourterminal shunt is coupled to the primary winding at the endcorresponding to the first end of the first secondary winding and asecond current lead of the four terminal shunt is coupled to the otherend of the primary winding via the load; a first voltage lead of thefour terminal shunt at the same end as the first current lead of thefour terminal shunt is coupled to the external circuit; and a secondvoltage lead of the four terminal shunt is coupled to a second voltagelead of the four terminal shunt and to the external circuit.
 7. Thecurrent source according to claim 1, further comprising a third fourterminal shunt coupled in series with a load across the primary winding;and the processing circuit comprises a first four terminal shunt and asecond four terminal shunt; wherein a first current lead of the firstfour terminal shunt is coupled to a first predetermined end of the firstsecondary winding and a second current lead of the first four terminalshunt is coupled to the other end of the first secondary winding via thealternating current source; a first current lead of the second fourterminal shunt is coupled to a first end of the second secondary windingcorresponding to the first end of the first secondary winding and asecond current lead of the second four terminal shunt is coupled to theother end of the second secondary winding; a first current lead of thethird four terminal shunt is coupled to the primary winding at the endcorresponding to the first end of the first secondary winding and asecond current lead of the third four terminal shunt is coupled to theother end of the primary winding via the load; a first voltage lead ofthe first four terminal shunt at the same end as the first current leadof the first four terminal shunt is coupled to the external circuit; afirst voltage lead of the second four terminal shunt at the same end asthe first current lead of the first four terminal shunt is coupled tothe external circuit; a first voltage lead of the second four terminalshunt at the same end as the second current lead of the second fourterminal shunt is coupled to the external circuit; the second voltageleads of the first and second four terminal shunts are coupled together;a first voltage lead of the third four terminal shunt at the same end asthe first current lead of the third four terminal shunt is coupled tothe external circuit; and a second voltage lead of the second fourterminal shunt is coupled to a second voltage lead of the third fourterminal shunt and to the external circuit.
 8. The current sourceaccording to claim 1, further comprising a third four terminal shuntcoupled in series with a load across the primary winding; and theprocessing circuit comprises a first four terminal shunt and a secondfour terminal shunt; wherein a first current lead of the first fourterminal shunt is coupled to a first predetermined end of the firstsecondary winding and a second current lead of the first four terminalshunt is coupled to the other end of the first secondary winding via thealternating current source; a first current lead of the second fourterminal shunt is coupled to a first end of the second secondary windingcorresponding to the first end of the first secondary winding and asecond current lead of the second four terminal shunt is coupled to theother end of the second secondary winding; a first current lead of thethird four terminal shunt is coupled to the primary winding at the endcorresponding to the first end of the first secondary winding and asecond current lead of the third four terminal shunt is coupled to theother end of the primary winding via the load; a first voltage lead ofthe first four terminal shunt at the same end as the first current leadof the first four terminal shunt is coupled to ground; the secondvoltage leads of the first and second four terminal shunts are coupledtogether; a first voltage lead of the third four terminal shunt at thesame end as the first current lead of the third four terminal shunt iscoupled to the external circuit; and a second voltage lead of the thirdfour terminal shunt is coupled to a first voltage lead of the secondfour terminal shunt and to the external circuit.
 9. The current sourceaccording to claim 1, further comprising a shield disposed between theprimary winding and each of the first secondary winding and the secondsecondary winding, wherein the shield is coupled to the control circuit.10. The current source according to claim 1, wherein at least one of:the current source forms part of a bridge for establishing a value of atleast one of a resistance and an inductance of a load; the controlsignal comprises a setting signal to establish a setting for at leastone of a current and a voltage to be generated and applied to a loadcoupled to the primary winding and a feedback signal established independence upon the output of the processing circuit; and the dual stagecurrent transducer comprises at least one of: a first magnetic core inthe form of a hollow toroid around which the primary winding and thefirst secondary winding are wound and a second magnetic core in the formof a toroid embedded within the first magnetic core around which thesecond secondary winding is wound; a first magnetic core in the form ofa toroid disposed in a plane parallel to a second magnetic core also inthe form of toroid wherein the primary winding and the first secondarywinding are wound around the first magnetic core and the second magneticcore and the second secondary winding is wound around the secondmagnetic core; and three magnetic cores in the form of toroid disposedin planes parallel to each other wherein the primary winding and thefirst secondary winding are wound around all three magnetic cores andthe second secondary winding is wound around the middle magnetic core ofthe three magnetic cores.
 11. A method of providing a current sourcecomprising: providing a dual stage current transducer comprising atleast a primary winding, a first secondary winding and a secondsecondary winding; providing an alternating current source disposedbetween the first secondary winding and a processing circuit forgenerating a current to generate a first electrical output signal acrossthe primary winding to be coupled to a load and comprising a controlport for receiving a control signal for controlling the alternatingcurrent source; providing a processing circuit coupled to the firstsecondary winding and the second secondary winding for providing asecond electrical output signal relating to the current generated by thealternating current source to an external circuit; and providing acontrol circuit coupled to the processing circuit for receiving thesecond electrical output signal and generating the control signal independence upon at least the second electrical output signal.
 12. Themethod of providing a current source according to claim 11, wherein theprocessing circuit comprises providing a first four terminal shunt and asecond four terminal shunt; wherein a first current lead of the firstfour terminal shunt is coupled to a first predetermined end of the firstsecondary winding and a second current lead of the first four terminalshunt is coupled to the other end of the first secondary winding via thealternating current source; a first current lead of the second fourterminal shunt is coupled to a first end of the second secondary windingcorresponding to the first end of the first secondary winding and asecond current lead of the second four terminal shunt is coupled to theother end of the second secondary winding; a first voltage lead of thefirst four terminal shunt at the same end as the first current lead ofthe first four terminal shunt is coupled to the external circuit; afirst voltage lead of the second four terminal shunt at the same end asthe second current lead of the second four terminal shunt is coupled tothe external circuit; and the second voltage leads of the first andsecond four terminal shunts are coupled together.
 13. The method ofproviding a current source according to claim 11, wherein providing theprocessing circuit comprises providing a current comparator having afirst input, a second input, and an output with a resistor disposedbetween the first input of the current comparator and the output of thecurrent comparator which is also coupled to the external circuit;wherein the first input of the current comparator is coupled to a firstpredetermined end of the first secondary winding and a firstpredetermined end of the second secondary winding corresponding to thefirst end of the first secondary winding; the second input of thecurrent comparator is coupled to the other end of the second secondarywinding to the other end of the first secondary winding via thealternating current source.
 14. The method of providing a current sourceaccording to claim 11, wherein providing the alternating current sourceis selected from the group comprising a stabilized alternating currentsource, an adjustable alternating current source, a programmablealternating current source and a controllable alternating currentsource.
 15. The method of providing a current source according to claim11, further comprising; providing a third four terminal shunt coupled inseries with a load across the primary winding; wherein providing theprocessing circuit comprises providing a first four terminal shunt and asecond four terminal shunt; a first current lead of the first fourterminal shunt is coupled to a first predetermined end of the firstsecondary winding and a second current lead of the first four terminalshunt is coupled to the other end of the first secondary winding via thealternating current source; a first current lead of the second fourterminal shunt is coupled to a first end of the second secondary windingcorresponding to the first end of the first secondary winding and asecond current lead of the second four terminal shunt is coupled to theother end of the second secondary winding; a first current lead of thethird four terminal shunt is coupled to the primary winding at the endcorresponding to the first end of the first secondary winding and asecond current lead of the third four terminal shunt is coupled to theother end of the primary winding via the load; a first voltage lead ofthe first four terminal shunt at the same end as the first current leadof the first four terminal shunt is coupled to ground and the externalcircuit; a first voltage lead of the second four terminal shunt iscoupled to a second voltage lead of the second terminal shunt at thesame end as the second current lead of the second terminal shunt and theexternal circuit; a first voltage lead of the second four terminal shuntat the same end as the second current lead of the second four terminalshunt is coupled to the external circuit; the second voltage leads ofthe first and second four terminal shunts are coupled together; a firstvoltage lead of the third four terminal shunt at the same end as thefirst current lead of the third four terminal shunt is coupled to theexternal circuit; and a second voltage lead of the second four terminalshunt is coupled to a second voltage lead of the third four terminalshunt and to the external circuit.
 16. The method of providing a currentsource according to claim 11, wherein providing the processing circuitcomprises providing a current comparator having a first input, a secondinput, and an output, providing a resistor disposed between the firstinput of the current comparator and the output of the current comparatorwhich is also coupled to the external circuit, and providing a fourterminal shunt coupled in series with a load across the primary winding;wherein the first input of the current comparator is coupled to a firstpredetermined end of the first secondary winding and a firstpredetermined end of the second secondary winding corresponding to thefirst end of the first secondary winding; the second input of thecurrent comparator is coupled to the other end of the second secondarywinding and to the other end of the first secondary winding via thealternating current source; a first current lead of the third fourterminal shunt is coupled to the primary winding at the endcorresponding to the first end of the first secondary winding and asecond current lead of the four terminal shunt is coupled to the otherend of the primary winding via the load; a first voltage lead of thefour terminal shunt at the same end as the first current lead of thefour terminal shunt is coupled to the external circuit; and a secondvoltage lead of the four terminal shunt is coupled to a second voltagelead of the four terminal shunt and to the external circuit.
 17. Themethod of providing a current source according to claim 11, furthercomprising providing a third four terminal shunt coupled in series witha load across the primary winding; wherein the processing circuitcomprises a first four terminal shunt and a second four terminal shunt;a first current lead of the first four terminal shunt is coupled to afirst predetermined end of the first secondary winding and a secondcurrent lead of the first four terminal shunt is coupled to the otherend of the first secondary winding via the alternating current source; afirst current lead of the second four terminal shunt is coupled to afirst end of the second secondary winding corresponding to the first endof the first secondary winding and a second current lead of the secondfour terminal shunt is coupled to the other end of the second secondarywinding; a first current lead of the third four terminal shunt iscoupled to the primary winding at the end corresponding to the first endof the first secondary winding and a second current lead of the thirdfour terminal shunt is coupled to the other end of the primary windingvia the load; a first voltage lead of the first four terminal shunt atthe same end as the first current lead of the first four terminal shuntis coupled to the external circuit; a first voltage lead of the secondfour terminal shunt at the same end as the first current lead of thefirst four terminal shunt is coupled to the external circuit; a firstvoltage lead of the second four terminal shunt at the same end as thesecond current lead of the second four terminal shunt is coupled to theexternal circuit; the second voltage leads of the first and second fourterminal shunts are coupled together; a first voltage lead of the thirdfour terminal shunt at the same end as the first current lead of thethird four terminal shunt is coupled to the external circuit; and asecond voltage lead of the second four terminal shunt is coupled to asecond voltage lead of the third four terminal shunt and to the externalcircuit.
 18. The method of providing a current source according to claim11, further comprising providing a third four terminal shunt coupled inseries with a load across the primary winding; wherein the processingcircuit comprises a first four terminal shunt and a second four terminalshunt; a first current lead of the first four terminal shunt is coupledto a first predetermined end of the first secondary winding and a secondcurrent lead of the first four terminal shunt is coupled to the otherend of the first secondary winding via the alternating current source; afirst current lead of the second four terminal shunt is coupled to afirst end of the second secondary winding corresponding to the first endof the first secondary winding and a second current lead of the secondfour terminal shunt is coupled to the other end of the second secondarywinding; a first current lead of the third four terminal shunt iscoupled to the primary winding at the end corresponding to the first endof the first secondary winding and a second current lead of the thirdfour terminal shunt is coupled to the other end of the primary windingvia the load; a first voltage lead of the first four terminal shunt atthe same end as the first current lead of the first four terminal shuntis coupled to ground; the second voltage leads of the first and secondfour terminal shunts are coupled together; a first voltage lead of thethird four terminal shunt at the same end as the first current lead ofthe third four terminal shunt is coupled to the external circuit; and asecond voltage lead of the third four terminal shunt is coupled to afirst voltage lead of the second four terminal shunt and to the externalcircuit.
 19. The method of providing a current source according to claim11, further comprising providing a shield disposed between the primarywinding and each of the first secondary winding and the second secondarywinding; wherein the shield is coupled to the control circuit.
 20. Themethod of providing a current source according to claim 11, wherein atleast one of: the current source forms part of a bridge for establishinga value of at least one of a resistance and an inductance of a load; thecontrol signal comprises a setting signal to establish a setting for atleast one of a current and a voltage to be generated and applied to aload coupled to the primary winding and a feedback signal established independence upon the output of the processing circuit; and the dual stagecurrent transducer comprises at least one of: a first magnetic core inthe form of a hollow toroid around which the primary winding and thefirst secondary winding are wound and a second magnetic core in the formof a toroid embedded within the first magnetic core around which thesecond secondary winding is wound; a first magnetic core in the form ofa toroid disposed in a plane parallel to a second magnetic core also inthe form of toroid wherein the primary winding and the first secondarywinding are wound around the first magnetic core and the second magneticcore and the second secondary winding is wound around the secondmagnetic core; and three magnetic cores in the form of toroid disposedin planes parallel to each other wherein the primary winding and thefirst secondary winding are wound around all three magnetic cores andthe second secondary winding is wound around the middle magnetic core ofthe three magnetic cores.